In general, a fingerprint matching system always requires a fingerprint image reproduction device which, prior to executing fingerprint matching processing, extracts fingerprint ridges from a fingerprint image as the matching target and clearly reproduces a fingerprint ridge images showing the shapes of the fingerprint ridges.
In that case, normally, when extracting the fingerprint ridges, first, a region of the fingerprint image to be the target is specified, the distances between the neighboring ridges are measured, the streaked pattern formed by the ridges is handled as a plane wave, and “Short-time Fourier Transform (referred to as “STFT” hereinafter)” or discrete wavelet transform is used thereupon to execute processing for clarifying the fingerprint ridges contained within the fingerprint image. This makes it possible in the region of low ridge quality where the ridge shape is unclear within the fingerprint image to interpolate the fingerprint ridges and to execute extraction of a minute ridge structure of smaller inter-ridge distances than a specific value, etc.
It is possible to interpolate the ridge shape regarding the part where the ridge in the fingerprint image is intermittent in a case where the ridge image is extracted by using Fourier transform on the fingerprint image. On the other hand, in a minutia such as an end point where the fingerprint ridge is intermittent or a bifurcation where the fingerprint image is branched, the ridges may be connected in some cases due to the property of Fourier transform so that the minutia may become ruined.
Further, also in a case where the ridge image is extracted by using analysis of the fingerprint image by using discrete wavelet transform, the ridge shape cannot be captured and presented accurately in a fingerprint region where a region of a minute characteristic with a small inter-ridge distance and a region with a large inter-ridge distance are mixed.
As a related technique thereof, there is known a system which executes extraction of the ridge shape by using discrete wavelet transform through performing frequency tuning for each block region set within a fingerprint image for capturing the inter-ridge distance of the minute structure within the fingerprint image (Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication 2004-127062
However, while it is possible with the related technique disclosed in Patent Document 1 described above to acquire the distances regarding the entre ridges within the target block region, it is not possible to accurately capture the changes in the distances between the ridges in a pixel level, i.e., it is not possible to accurately capture the minute structure of the ridges in the region where the minute ridges of different inter-ridge distances are contained.
In particular, the minute structure of the ridges cannot be captured often in a case where the regions of wide inter-ridge distance (ridge pitch) and the regions of narrow inter-ridge distance exist in a mixed manner. Thus, the ridge structure is spoiled in the ridge image reproduced based on the result of such analysis.
Further, the accuracy of the fingerprint matching processing which performs the short-time Fourier transform (STFT) and the discrete wavelet transform described above depends on the extraction accuracy of the minute minutiae. Therefore, when the ridge structure is spoiled, the accuracy of the fingerprint matching becomes extremely deteriorated.
It is therefore an object of the present invention to provide a fingerprint ridge image synthesis system, a fingerprint ridge image synthesis method, and a program thereof, which can improve the shortcomings of the above-described related technique and accurately extract the ridge shapes contained in unclear fingerprint images thereby to synthesize the fingerprint image in a relatively clear manner.